Purified antibodies specific for bovine neurophysins I and II have been obtained from rabbit antisera against the proteins conjugated to polyalanylpolylysine, with final purificatons of these antibodies by affinity chromatography on immobilized neurophysin matrices. The high level of immunological cross-reactivity of neurophysins I and II was verified by the affinity chromatographic technique, indicating the extensive structural similarity between these species. Purified neurophysin antibodies were bound to Sepharose to generate an immunoadsorption matrix. This matrix binds not only native neurophysin but also a protein derivative in which the hormone binding site is blocked. This matrix will be used mainly to try to isolate a postulated neurophysin precursor from hypothalamic tissue or other appropriate source. Preliminary results indicated the presence in hypothalamic extracts of species, which bind to the immunoadsorbant, of molecular weight higher than that of neurophysin. A program has been initiated to prepare antibodies against thermolysin, as well as some large fragments of this protein. This study is designed as an approach to understanding the conformational propensities of isolated protein domains.